This chapter discusses the properties of atomic defects in molybdenum (Mo). Molybdenum single crystals can be prepared in high-purity condition by electron-beam zone melting. However, it must be noted that W solute atoms, usually present in a range of 100···300 at ppm cannot be removed by this technique. The preparation of material with lower W impurity contents requires special techniques. Interstitial impurities are less important than in V,Nb,Ta, due to lower solubilities and lower reactivities. Strong support for vacancy migration in stage III has been obtained by nuclear-probe techniques. Properties of Frenkel defects, self-interstitial atoms (SIAs) and vacancies in molybdenum are tabulated. Resistivity recovery of molybdenum following thermal-neutron damage at 3.6K, resistivity recovery of irradiated molybdenum, resistivity recovery of single crystal molybdenum following electron irradiation at 4K, resistivity recovery of molybdenum following neutron irradiation near room temperature, and resistivity recovery of neutron irradiated molybdenum are also illustrated.